Bill Fitzmaurice

Allison Effect, the cancellation from the reflection off the rear wall. To prevent it the subs must be close to the wall, the mains far from it. When both are the same distance away it's unavoidable. See above. Since you can put the sub anywhere that relieves some of the space concerns. By dint of using a bass combo as a sub it is a sub, and the same placement rules apply.

Because the sub probably has more available output from 60-100Hz than the tops. Crossing lower than 100Hz with tops that are on stands also brings floor bounce cancellation into the equation. No, as that results in directionally locatable frequencies coming from the subs. Then you lose the ability to put the subs anywhere, which should never be below the mains unless there's no alternative.

Once again, port shape ...doesn't matter, though narrow slots should be avoided. That's as far as sound is concerned. From a construction standpoint I use corner ports, as they do double duty as bracing for the baffle, top, bottom and sides.

That being the case you don't need the subwoofer bottom cab of a Bose clone, even as meager as it is. You do need two speakers, for even coverage. Size being the issue a pair of powered 8" or 10" loaded mains will do, and you'll get a lot more value for the money with those than with mini line arrays, which tend to be terribly over priced. Another advantage is you can use them with your existing sub when the need arises, and add them to your regular system for outdoor gigs.

Bracing is seldom unnecessary, unless you're building with 36mm plywood. My personal cabs are built from 3mm and 6mm plywood, which is possible because they use necessary bracing. Sure, if you want state of the art circa 1984. 😲

Port shape (duct actually, the name 'port' is incorrectly applied to ducts, but it's part of the lexicon now) doesn't matter, though narrow slots should be avoided. The area of those ports looks to be far too small.

It would also result in chuffing, as the port area would be too small.

The cabinet is simply far too large, so it doesn't provide the necessary restorative force to prevent over-excursion. That mainly relates to the driver Vas. For the K140 that's 297 L. For the 3015 it's 153 L.

That diagram shows the 3015LF, not the 3015. The 3015 works best in 70 liters/2.5 cu ft (net) with 200 cm2 port area 35cm long for 45Hz tuning.

In 60 years of playing I never had an amp failure, so it wasn't worth it to me to carry another piece of the heavy iron that ruled for the first 40 years or so. But given how small good amps can be had today if I was to keep a spare it would be one of those midget amps rather than a pedal.

Without detailed specs on the woofer and tweeter one cannot construct an ideal crossover. If you have the impedance of the drivers you can use an off the shelf crossover that will still be better than what you have, for instance https://www.parts-express.com/Eminence-PX-BII-3K5-2-Way-Crossover-Board-3-500-Hz-290-636?quantity=1

The APT drivers were changed years ago from a proprietary non-standard attachment to a standard 1 3/8" screw on. The horn in your picture looks like the old design. A picture of your driver would confirm if it's the new design.

Hum is almost always caused by grounding/earthing issues with the wiring and there's seldom anything you can do about it.

That's not a crossover. A crossover splits the low and high frequencies, sending the lows to the woofer, the highs to the tweeter. That's just a high pass filter, and a very poor one at that. It's what's called 1st order, meaning that it attenuates below its knee frequency at a rate of 6dB per octave, which is totally inadequate. I never use less than 3rd order high pass filters, which attenuate below the knee frequency at a rate of 18dB/octave. That makes them over ten times more effective in reducing low frequency content to the tweeter that leads to high distortion and blown tweeters.

Search 'sound analyzer app'. I can't recommend one as I don't have iPhone.

I get whatever tonal changes I want with technique and occasionally the tone control on the bass. I seldom touch the amp on the fly. I do adjust the amp at sound check because close to half the result is dictated by the room. I always go out onto the dance floor to hear what it sounds like there, and if there's an FOH I check it there as well. I never trust the sound man to get it right, unless said sound man is me.

The reason I said to take the RTA playing an open A is that your tone is a product of the amp EQ and the speaker. By using pink noise with the amp EQ flat you remove the amp EQ from the equation. You want the PA EQ to not only duplicate as much as possible the effect of the speaker but also the effect of the amp EQ. There's also the matter of whether the amp is actually flat when the EQ is set to flat. Most amps have built in voicing, so flat EQ may not give a flat result.

Most top acts tend to over-use subs as well. I'd say three out of four FOH mixers have no idea how to mix bass. The fourth is a bass player. ☺️

Look at the frequency response chart here, which is a typical high quality bass driver. Note the midrange that peaks at 2.5kHz. https://eminence.com/products/kappalite_3012ho#frequency-response

PA subs go as much as an octave lower than bass cabs, while PA mains don't have the rising midrange typical of bass cabs. Try high passing the desk bass channel at 60 to 80Hz, if it has that capability, or cutting back on the bass EQ if it doesn't, while boosting the mids in the 2 to 2.5kHz range. If you want to be more precise get this app for your phone. https://play.google.com/store/apps/details?id=com.dom.audioanalyzer&hl=en_US&pli=1 You can use it to see the frequency response of your rig. Play an open A and take a picture of the result, C weighting, slow response. Save it and compare it to the same test through the PA. Comparing the two will show where the PA EQ needs to be adjusted.

Longer throw, via the speakers being in the nearfield condition so the level drops by 3dB per doubling of distance rather than 6dB, only occurs when the line is at least three wavelengths high. That's easy enough at 5kHz or higher, not so much at 100 Hz, where three wavelengths is about ten meters. Wider horizontal dispersion, twice as wide above roughly 300 Hz, is the main advantage with other than Grateful Dead Wall of Sound back lines.

You will, because it puts drivers closer to your ear level. It's one reason why 810s are so popular.

To put that into context for the same output level you need at least twice the cone displacement with bass as you do with guitar. To realize twice the displacement requires double the voltage swing, which is four times the power. You may need as much as four times the displacement, which is four times the voltage swing, which is sixteen times the power.

Surface area by itself means next to nothing. What matters is area x excursion, ie., displacement. BF lists it, Ashdown doesn't. In any event you're not going to get anywhere near what the BF you have is capable of with 30 watts. 300 would be more appropriate.

Attenuation is likely required, but using series resistors, which is what it appears to be, is the wrong way to do it. An LPad is the right way, be it variable or using a fixed value with series/parallel resistors, just as using an NPE cap as a basic high pass filter is the wrong way to divide the frequencies and provide proper protection for the tweeter. Where the enclosure is concerned you can get a decent result with that driver, but because of the massive Vas not from 95L. 200L with 35Hz Fb is pretty good, but as the OP already has the box in hand that's probably not an option.